An Automatically Switched Transport Network (ASTN) architecture defines a network including a set of control functions used to establish and release connections across a transport network. In general, the ASTN architecture includes three planes; a control plane, a management plane and a transport plane. The control plane supports both switched and soft permanent connections by efficient configuration of connections within the transport plane. The management plane provides the Operational, Administration and Management (OAM) functions for the control and transport plane. The transport plane provides bidirectional or unidirectional transfer of user information, from one location to another. It can also provide transfer of some control and network management information.
In the ASTN architecture, communication between two end points is performed using calls. A call contains call data and connection data including an end-to-end path. A call can have more than one connection between the head and tail end nodes of the call. A connection is a path between the head and tail end nodes of the call. The connection consists of several sub-network connections, where a sub-network connection could be, for example, a switch matrix connection in one network element in the path. Thus the connection is a concatenation of link connections and sub-network connections that allow the transport of user information between head and tail end nodes of a call. Calls between endpoints may be initiated either automatically (by a router) or alternatively manually by a network operator through user entry at an Operation Administration and Maintenance (OAM) station.
The control plane provides a network operator with the ability to offer a user calls with a selectable class of service (CoS) (e.g., availability, duration of interruptions, Errored Seconds, etc). Protection and restoration are ‘survivability mechanisms’ that are used by management plane to support the CoS requested by the user. Protection schemes protect a communication link by pre-calculating an alternate ‘protection’ path. In the event of a failure of the communication link, traffic is immediately switched from the working path to the protection path. In most cases protection paths are defined in layer one of the network architecture.
In contrast, the restoration mechanism identifies an alternate, restoration path between a head and tail node only after a failure occurs on the communication link. Restoration is typically performed at the network control layer. Both protection and restoration mechanisms have their advantages and disadvantages. One advantage of the protection mechanism is that the protection channel is immediately available in the event of need, and thus there is virtually no impact in traffic delivery when switching from the working to the protection channel. However, a drawback of the protection mechanism is that it requires installation of a protection channel for each APS defined working channel. In networks having large numbers of connections, storing a protection channel for each working channel may not be feasible. Restoration is a solution that overcomes these problems, because restoration paths are only generated when needed, and deleted thereafter. However, the transfer of a call resulting from a failure or maintenance of a link in the connection associated with the call is delayed by the need to compute and signal a new path for a connection.